Following the approval of the space shuttle space program a need arose for amorphous silica fiber having excellent resistance to devitrification at temperatures up to 2500.degree. F. and for exposure times at this temperature of up to 4 hours. It has long been recognized that fibers of substantially pure silica can be produced by forming fibers from a glass such as type E glass having softening and melting characteristics suitable for convenient fiber formation followed by leaching to remove substantially all of the nonsilica components from the fibers by immersing the E glass fibers in an aqueous solution of sufficient acidity to extract the acid soluble components. Improved fibers of this general type are disclosed in U.S. Pat. No. 3,687,850, but these fibers will not meet the above described stringent devitrification resistance requirements because of the relatively high alumina contents of 4 to 8% and the relatively high contents of other nonsilica materials such as calcium oxide, barium oxide, magnesium oxide, and boron oxide.
It has also been known to make silica fibers from glass compositions containing mostly soda and silica, for example, see U.S. Pat. Nos. 3,092,531 and 3,560,777. The fibers described in the latter patent were useful to temperatures of only about 2200.degree. F. because of a relatively high impurity level. Attempts to make amorphous silica fibers having excellent resistance to devitrification at temperatures as high as 2500.degree. F. according to the process disclosed in U.S. Pat. No. 3,092,531, but modified for treatment of loose fiber instead of fibrous mats, resulted in fibers having a high degree of nonuniformity and thus a very low percentage, e.g., 15 to 20% acceptance level based on devitrification resistance at 2500.degree. F. for 4 hours.
Several techniques of leaching were developed in an attempt to make the desired silica fibers. In one technique sodium silicate glass fibers made using ordinary glass sand were placed into an open tank. An acid solution was then added to the tank and the tank was heated with burners directly beneath the tank to bring the solution up to the proper temperature for leaching. Once the acid solution was brought up to the desired temperature, it was maintained at this temperature until the leaching process was completed with the batch in the tank being stirred occasionally to break up the fiber clumps and redistribute the fibers within the tank. After the desired silica content was obtained, the acid solution was drained from the tank and a rinsing operation was commenced. Once the fibers had been rinsed to the necessary degree, they were removed from the tank and pressed into cakes. This pressing operation eliminated about 80% of the water from the fibers. The cakes were then placed in an oven and dried. Afterwards, the outside surfaces of the cakes were trimmed off and the cakes were broken up to form fiber clumps for shipment. Fiber made according to this process proved to be very nonuniform in impurity level and in devitrification resistance and was unacceptable for the intended use.
In an attempt to improve uniformity a process described in U.S. patent application Ser. No. 391,394, filed Aug. 24, 1975 and now abandoned, was developed (hereinafter "perking process"). In this process the fiber was put into a perforated basket residing in the upper half of a leach tank. Hot acid was recirculated in the tank, passing down through the fiber and rising up through an inverted funnel whose stem extended into the middle of the basket area. This method was unsuccessful for several reasons. First, the perking action (acid recirculation) did not begin until the temperature of the acid solution reached 140.degree. F. Up to this time, sodium was being extracted from the fiber, but remained in the immediate vicinity of the fiber. If the slurry was not periodically moved, enough sodium salt could accumulate to recombine with the fiber and form a cementitious mass of bonded filaments. These conglomerates were often too large to leach thoroughly. Second, stirring could alleviate the consolidation problem, but was difficult to do well by hand because of the stiffness of the fiber and costly to do by machine because of the interference of the center stem. Third, once the unit began to perk, lids were required for the tank to protect the tank operator. Stirring was needed during perking because, although a perforated lid was set on top of the basket to distribute the acid uniformity over the fiber, channeling inevitability occurred as the acid filtered down through the fiber, causing some pockets of fiber to go incompletely treated. The installation of lids prevented even hand-stirring unless the entire unit was shut down. Finally, the perk tank could not be operated in a controllable or reproducable manner.
In an attempt to avoid the problems associated with the perking process, a process disclosed in U.S. patent application Ser. No. 391,395, filed Aug. 24, 1973 and now abandoned (hereinafter '395) was developed. In this process loose sodium silicate fibers, made using ordinary glass sand, were placed in a perforated basket resting in the upper half of a leaching tank. The acid solution was added to the tank until the fibers in the perforated basket were barely covered. Burners below the tank were ignited to heat the acid solution to the desired temperature and immediately circulation of the acid solution from the bottom of the tank back to the top of the perforated basket was begun. This was accomplished using a pump and line external of the leaching tank. The acid solution was added back to the basket of fibers using either a rotating spray manifold or by dumping the acid solution onto the top of a perforated lid allowing the acid solution to drain down through holes in the lid into the fibrous mass. As soon as the acid solution reached the desired temperature the burners were used only to maintain the desired temperature during the leaching period. Following the leaching period the fibers were rinsed and processed in the same manner as described above. While the '395 process eliminated most of the problems inherent in the perking process, the resultant fibers showed a high degree of nonuniformity and only about 15-20% of the lots of fibers made according to this technique displayed the necessary devitrification resistance at 2500.degree. F. that was required.
It was also discovered that when the average level of alumina in the leached fibers dropped below about 0.16%, by weight, the fiber displayed good devitrification resistance, but when the alumina content was above about 0.20%, substantial devitrification nearly always occurred in a 2500.degree. F. test. It was also determined that the alumina content in the leached fibers of the prior art varied considerably from levels below 0.16% to levels well above 0.2% within the same batch.
Since the alumina impurity was being introduced into these fibers by the use of industrial grade glass sand in the fiber manufacturing process, it was decided that the problem could be solved by using pure silica in place of the glass sand. Fibers were made using CAB-O-SIL, a high purity silica product of the Geofrey L. Cabot, Inc., and leached according to the process of '395. While having a very low alumina content of below about 0.025%, by weight, and displaying satisfactory devitrification resistance, the very fine particle size of the CAB-O-SIL, and other high purity silica materials, would require pretreatment, e.g., pelletizing, prior to melting for the production of the glass fibers to prevent excessive dust problems and losses. Also, the relatively high cost of these pure silica materials compared with ordinary glass sand would be a significant disadvantage in the manufacture of silica fibers.
It has also been discovered, as evidenced by U.S. patent application Ser. No. 555,777, filed Mar. 6, 1975 and now abandoned (hereinafter '777), that if the silica raw material used to make the precursor sodium silicate fibers is properly selected based on the level of nonsilica impurities it contains, and if the fibrous mass is periodically gently stirred and fluffed up during the recirculating acid solution leaching and the rinsing cycles silica fibers can be produced having excellent resistance to devitrification and relatively low shrinkage and distortion up to 2500.degree. F. Using the process of the '777 invention, relatively low cost silica raw materials can be used to produce a silica fiber having alumina contents above about 0.025% and below about 0.2%. The titanium dioxide content, which is thought to be significant to devitrification resistance, is preferably lower than about 0.02% and preferably below about 0.01% in the final silica fiber.
The apparatus of '777 differed from the apparatus described in '395 by utilizing a mechanical stirrer or agitator in addition to or in combination with the means used to distribute the acid solution to the top surface of the fibrous mass in the perforated basket.